1. Field of the Invention
The invention relates to a method of irradiation welding of two thermoplastic components by producing a weld seam in an area of joining between the absorptive and transmissive component parts by means of an energy beam, in particular laser beam. The invention further relates to an apparatus for putting the method into practice.
2. Background Art
For better understanding of the invention, the fundamentals of laser irradiation welding of plastics are going to be explained, taken in conjunction with prior art systems.
In laser irradiation welding of plastics, laser irradiation penetrates the first weld seamed part that is turned towards the beam source and is being absorbed by the second weld seamed part by only minor depth of penetration of the surface and converted into heat. By heat conduction the transmissive weld seamed part is equally being melted.
In so-called quasi simultaneous welding, the laser beam is being run rapidly along the weld contour for several times. Quasi simultaneous welding of plastic components has established as a very common method in laser irradiation welding. U.S. Pat. No. 6,444,946 B1 describes a corresponding method, specifying that the entire weld seam is being plasticized substantially in a single cycle after being heated to an “intermediate temperature” by a plurality of cycles. The principal idea of this method consists in the weld seam being heated as uniformly as possible; any spatial temperature gradients along the weld seam are not desired.
According to this document, in case of a closed weld seam, all areas of the weld seam are in a solid phase or all sectional areas are simultaneously in a plasticized condition, uniform melting of the seam taking place by the action of the joining pressure.
This welding method has the following drawbacks:                Current examinations of the working mechanism of plastics welding have shown that high clamping pressure positively affects the welding result. The assumption is that high contact periods of the parts being joined, accompanied with correspondingly high pressure, work in favour of molecular exchange processes (diffusion). Additionally, high clamping pressures improve the thermal contact of the parts being joined and accelerate the heat conduction into the top layer. The load that may act on a component part is as a rule limited, because damages of the part will produce easily. With the clamping force that is applied in a joining method according to U.S. Pat. No. 6,444,946 B1 spreading uniformly across the entire weld seam, given the fact that, without obstruction to expansion, the weld seam would stay nearly level by simultaneous plasticizing and the inferior temperature gradients, the locally produced clamping pressure decreases. With the effects of process acceleration of the high clamping pressure not being exploited, the efficiency, and thus economic profitability, of the welding method decrease.        In connection with quasi simultaneous welding, so-called weld seam-run monitoring is frequently used as a method of process diagnostics. In this case, the length is measured, by which the parts move towards one another under the action of joining pressure and with the seam plasticized. As a rule, this takes place by way of detection of the motion of the movable clamping plate. Drawbacks reside in that weld seam interruptions cannot be detected, because they do not affect the motion of the clamping plate, provided the spatial expansion of seam interruption is insignificant as compared to the entire seam contour, which is the rule.        Heat conduction, which confers energy from the absorbtive part being joined on the transmissive part being joined, is strongly time-dependent. For the transmissive top layer to melt, which is necessary for integral material joining, both parts being joined must stay in thermal contact for a certain time. During this time, due to the nearly uniform plasticizing of the weld seam, molten polymer compound is displaced from the area of joining by the joining pressure. The result is undesired escape of energy from the area of welding that comprises the heated polymer compound, which is accompanied with reduced process efficiency. Additionally, displaced polymer compound may flow into component areas where it is not desired for optical or functional reasons.        Notches in the absorbtive part being joined negatively affect the welding process. The notch can only be filled with polymer compound when the adjacent areas have been plasticized and a sufficient amount of polymer compound has been displaced for the notch to be closed. Only then the transmissive joining part above the notch can be supplied with energy by thermal contact via heat conduction. In addition to this negative effect, the polymer at the bottom of the notch can be damaged thermally, because it is heated as are intact areas of the weld seam seam, while however only little thermal energy is led by heat conduction to the cold top layer.        
In contour welding the weld seam is being plasticized locally, the integral joint occurring directly after a single irradiation job. Consequently this process is accompanied with some restrictions too:                There is no setting motion towards each other of the two parts being joined, which is why any geometric compensation for component tolerances is not possible.        Since the top layer of the joint is being plasticized mainly by heat conduction from the bottom layer, integral joining will take a certain time. For the polymer of the absorbtive bottom layer not to be damaged, the maximally employable laser energy density is limited upwards. Both circumstances explain why only inferior feed velocities can be used in contour welding, which is accompanied with prolonged process times than in quasi simultaneous welding.        